Mass flow measuring or controlling devices are for example known from document DE 10 2004 019 521 A1 and are intended to reliably measure or control flow rates of gases or liquids with high precision also over a relatively large flow range.
Mass flow measuring or controlling devices which are based on a thermal measuring principle usually comprise a sensor element which is arranged on a sensor support part and adjoins a flow channel.
There are two basic variants of mass flow measuring or controlling devices of this type with respect to the flow channel. The design mostly used so far provides that a main flow channel is provided in the housing, from which a bypass channel is branched off. The sensor element measures at the bypass channel. In the region of the main flow channel, which in terms of flow is parallel to the bypass channel, a flow resistance section is present to generate a drop in pressure. The design of the mass flow measuring or controlling device alternative thereto does not provide any bypass channel. Rather, the entire fluid flow will flow past the sensor element.
In the embodiment having a bypass channel, the latter requires a certain minimum length which is due to the fact that it is arranged above a flow divider generating a drop in pressure, so that only part of the entire flow flows through the bypass channel. Furthermore, flow conditions which are as constant as possible are to be achieved in a measuring section arranged in the bypass channel.
For a constant measuring accuracy, it is decisive that the spatial arrangement between the sensor element and the measuring section in the flow channel remains unchanged over the service life of the mass flow measuring or controlling device.
In particular in case of low flow rates, for example at 100 ml/min up to 1 ml/min, it is difficult to achieve a high control accuracy of 1-2%, for example. A pressure dependence of the control accuracy having a particularly high impact in this region is determined. A pressure loading on the parts contacted by the medium in the device can undesirably lead to the fact that the sensor support part slightly moves away from the flow channel in the region of the sensor element by a bending of the sensor support part, which results in a degradation of the control accuracy.
It is therefore the object of the invention to provide a mass flow measuring or controlling device which does not include the drawbacks mentioned above, which is stable under compressive loading and which has a high control accuracy, also in the range of very low flow rates of up to 1 ml/min.